Silicon thin film transistors, systems, and methods of making same

ABSTRACT

Systems and methods of fabricating silicon-based thin film transistors (TFTs) on flexible substrates. The systems and methods incorporate and combine deposition processes such as chemical vapor deposition and plasma-enhance vapor deposition, printing, coating, and other deposition processes, with laser annealing, etching techniques, and laser doping, all performed at low temperatures such that the precision, resolution, and registration is achieved to produce a high performing transistor. Such TFTs can be used in applications such as displays, packaging, labeling, and the like.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/023,418, filed Jan. 24, 2008, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to the fabrication of circuits.More particularly, the present invention relates to silicon-based thinfilm transistors, systems, and methods of forming the same on flexiblesubstrates.

BACKGROUND OF THE INVENTION

Transistors are semiconductor devices, commonly used as an amplifier oran electrically controlled switch. The transistor is the fundamentalbuilding block of the circuitry in computers, cellular phones, and allother modern electronic devices. Because of its fast response andaccuracy, the transistor is used in a wide variety of digital and analogfunctions, including amplification, switching, voltage regulation,signal modulation, and oscillators. Transistors may be packagedindividually or as part of an integrated circuit.

Thin film transistors (TFT) are a special kind of field effecttransistor made by depositing thin films of a semiconductor active layeras well as the dielectric layer and metallic contacts over a supportingsubstrate. A common substrate is glass, because one of the primaryapplication of TFTs is in liquid crystal displays (LCDs). This differsfrom the conventional transistor where the semiconductor materialtypically is the substrate, such as a silicon wafer.

TFTs can be made using a wide variety of semiconductor materials. Acommon material is silicon. The characteristics of a silicon-based TFTdepend on the crystalline state of the silicon. That is, thesemiconductor layer can be either amorphous silicon, microcrystallinesilicon, or it can be polysilicon or single crystalline silicon. Othermaterials which have been used as semiconductors in TFTs includecompound semiconductors such as cadmium selenium (CdSe) and metal oxidessuch as zinc oxide. TFTs have also been made using organic materials(referred to as an organic TFT or OTFT).

There has been a growing trend to fabricate TFTs on flexible substrates,such as, for example, plastics and poly films. Such substrates are lowerin cost than glass and provide a wider range of applications, such as,for example, flexible displays, packaging, signage, labeling, and othersimilar applications. However, many difficulties arise in themanufacture of TFTs on flexible substrates, particularly plastics,particularly due to the high temperatures used in manufacturing TFTs.

Because a flexible substrate such as plastic or poly film cannotwithstand the high annealing temperature, deposition, patterning, dopingand other processes used in TFT fabrication must be completed underrelatively low temperatures. Chemical vapor deposition, and/or physicalvapor deposition (usually sputtering) are techniques often applied inthe fabrication of TFTs. However, desirable performance characteristics,such as high carrier mobilities, low leakage currents and thresholdvoltages, for high-performance applications such as, for example, foruse in liquid crystal displays, are difficult to achieve when lowprocessing temperatures are used. Processing temperatures (<150° C.)below those used for TFT fabrication on glass substrates must bemaintained for compatibility with low-cost plastic substrate materials.In general, superior TFT performance is achieved with higher-temperaturefabrication processes, because crystalline material can be deposited atthe higher temperatures, dopants can be activated at highertemperatures, and the quality of the critical gate-dielectric interface,which is highly sensitive to process temperature, can be controlled.

To attempt to address these and other issues, researchers in the fieldhave tried a number of different approaches. Laser annealing of siliconhas been studied for many years as a means for improving the crystallinenature of a deposited film. Typically, an amorphous silicon film isdeposited and a short pulse duration excimer laser is used to melt aportion of the silicon film. As the silicon cools between pulses,crystallization may occur resulting in crystal grains of various sizes.This approach has been used with success on both glass and plasticsubstrates.

Laser-induced doping of silicon is a promising method that has been usedto create conductive silicon film for use in thin film transistors. Oneimplementation of laser-induced doping is known as gas immersion laserdoping (GILD). In a GILD process, a laser pulse induces the melting ofsilicon in the presence of a precursor, or dopant, gas. Part of the gasspecies already chemisorbed or impinging on the melted silicon surfacesdiffuses into the molten silicon. As a result of the melting andsolidification cycle in the presence of the dopant gas, the dopant isincorporated into the silicon layer. Using this approach and/or similarapproaches, researchers have been able to achieve high dopantconcentrations (both N-type and P-type) and excellent spatial dopantprofiles, all while retaining highly crystalline silicon and low surfaceroughness. Details of this process can be found in: G. Kerrien et al.,App. Surf. Sc. 186 (2002) at 45-51, and A. Slaoui et al., J. Appl. Phys67 (10) 1990, p. 6197.

Even though advances have been made in creating polycrystalline thinfilms, doping silicon films at low temperature and fabricating TFTsusing these thin films, there remains a need for relatively low-costsystems and simplified methods for fabricating a high-performanceSi-based TFT on a flexible, plastic substrate.

SUMMARY OF THE INVENTION

The systems and methods of fabricating a silicon-based thin filmtransistor of the present disclosure overcome the inherent deficienciesdescribed above. The systems and methods of the present disclosureprovide for the fabrication of TFTs on flexible substrates at lowprocessing temperatures in both a self-aligned and non-self-alignedfashion. The TFTs of the present invention exhibit better performancecharacteristics, such as, for example, high mobility, high switchingfrequency, low operating voltage, and low threshold voltage whencompared to other methods for TFT fabrication on flexible substrates.The improved performance is also achieved at a significantly lower costcompared to existing approaches. Further, the methods of embodiments ofthe present invention include a combination of deposition and patterningtechniques that can reduce the number of processing or fabricationsteps, such as etching steps, patterning steps, and the like, fromtraditional fabrication of TFTs to reduce the cost of TFT fabricationand increase input, while maintaining or enhancing TFT performance.

In embodiments of the invention, a method for making self-alignedsilicon thin film transistors includes depositing a barrier material onat least a portion of a flexible substrate, forming a one siliconassembly on at least a portion of the barrier material, depositing adielectric material to form a gate dielectric on at least a portion ofthe at least one silicon assembly, depositing a gate electrode materialon at least a portion of the dielectric material to form at least onetransistor assembly, selectively patterning the gate electrode materialand the dielectric material, selectively doping areas of the at leastone transistor assembly thereby forming a source and a drain, andremoving excess doping material to form a self-aligned silicon thin filmtransistor. In one embodiment, the barrier material is printed on thesubstrate, rather than blanket deposited, forming a patterned barrierlayer. This patterned barrier layer can increase the flexibility of thetransistor, and is less prone to cracking than unpatterned barrierlayers. Further, a patterned barrier layer promotes better adhesion tothe substrate, especially during flexure of the substrate, to reducedelamination of the transistor from the substrate.

In other embodiments of the invention, the silicon layer is cleanedbefore the dielectric material is deposited on it. This clean caninclude a cleaning process such as an HF clean, followed by UV/O₂ orUV/O₂/F₂ process to form a thin, high quality SiO2 interface between thesilicon layer and the gate dielectric. This interface improveselectrical performance between the Si and the gate dielectric, andtherefore overall transistor performance.

Embodiments of the present invention can include one or more printingsteps. In addition to the option of printing the transistor layers, suchas the barrier layer, silicon layer, dielectric layer, gate electrodelayer, and/or dopant layer(s), masking layers used in one or moreetching steps can be printed to achieve tighter tolerances of thecomponents, without compromising transistor performance. One such methodof printing layers of the transistor and/or masking layers can includethe use of printing plates formed by traditional photolithographymethods, as described in U.S. Patent Application Publication No.2008/0092377 entitled “Patterned Printing Plates And Processes ForPrinting Electrical Elements,” incorporated herein by reference in itsentirety.

The above summary of the invention is not intended to describe eachillustrated embodiment or every implementation of the present invention.The figures and the detailed description that follow more particularlyexemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart according to an embodiment of the invention.

FIG. 2 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 3A is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 3B is a top view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 4 is a top view of a patterned silicon deposit according to anembodiment of the invention.

FIG. 5 is a top view of a patterned silicon deposit according to anembodiment of the invention.

FIG. 6A is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 6B is a top view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 7A is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 7B is a top view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 8A is a side view of fabricated TFTs according to an embodiment ofthe invention.

FIG. 8B is a top view of fabricated TFTs according to an embodiment ofthe invention.

FIG. 9A is a side view of individually encapsulated fabricated TFTsaccording to an embodiment of the invention.

FIG. 9B is a top view of individually encapsulated fabricated TFTsaccording to an embodiment of the invention.

FIG. 10 is a flow chart according to another embodiment of theinvention.

FIG. 11 is a side view of partially fabricated TFT s according to anembodiment of the invention.

FIG. 12 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 12A is a side view of partially fabricated TFTs according to analternative embodiment of the invention from FIG. 12.

FIG. 13A is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 13B is a top view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 14 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 15 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 16 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 17 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 18A is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 18B is a top view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 19 is a side view of fabricated TFTs according to an embodiment ofthe invention.

FIG. 19A is a side view of fabricated TFTs with doped source and drainregions according to an embodiment of the invention.

FIG. 20 is a flow chart according to another embodiment of theinvention.

FIG. 21 is a flow chart according to yet another embodiment of theinvention.

FIG. 22 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 23 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 24 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 25 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 26 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 27 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 28 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 29 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 30 is a side view of partially fabricated TFTs according to anembodiment of the invention.

FIG. 30A is a side view of partially fabricated TFTs according to analternative embodiment of the invention from FIG. 30.

FIG. 31 is a side view of partially fabricated TFTs according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

A silicon-based thin film transistor, according to various embodimentsdescribed herein, generally can be fabricated on flexible substrates,such as polymeric or plastic substrates or metal foil substrates such asstainless steel, using either a non-self aligned approach or aself-aligned approach. As defined herein, a self-aligned approach issuch that the gate electrode layer can be used as a mask that enablesone to align the gate with the source, drain, and channel of thetransistor without the need for additional or independent steps. Theapproach of this invention also improves the flexibility of the endproduct or TFT circuit by removing excess silicon and silicon dioxidewhere they are not needed. Both such approaches are depicted anddescribed herein.

Transistor devices or TFTs of the present invention generally comprisesource and drain electrodes, a semiconductor channel, dielectric, andgate electrode layers, arranged in any of a combination of ways tocreate a functional transistor. Patterned layers therefore can compriseconductive, semiconductive, and insulating materials printed orotherwise deposited with suitably formulated conductive, insulative, orsemiconductive ink, fluid, powder, particulate, or other depositioncompositions. T he methods of the present invention illustrate acompilation of steps that can be performed, although not necessarily inthe particular order as described, to build the TFTs of the presentinvention. The final TFT assembly is represented as a subassemblydefined as the resulting structure after each particular step, asdescribed below, as the TFT is fabricated. For method 100 of FIG. 1, thesubassemblies are represented at 50 in FIG. 2, at 52 in FIG. 3A, at 54in FIG. 6A, at 56 in FIG. 7A, at 58 in FIG. 8A, and at 60 in FIG. 9A.For method 300 of FIG. 10, the subassemblies are represented at 62 inFIG. 11, at 64 in FIG. 12, at 66 in FIG. 13A, at 68 in FIG. 14, at 70 inFIG. 15, at 72 in FIG. 16, at 74 in FIG. 17, and at 76 in FIG. 18A and78 in FIG. 19. For method 600 of FIG. 21, the subassemblies arerepresented at 80 in FIG. 22, at 82 in FIG. 23, at 84 in FIG. 24, at 86in FIG. 25, at 88 in FIG. 26, at 90 in FIG. 27, at 92 in FIG. 28, at 94in FIG. 29, at 96 in FIG. 30 and at 98 in FIG. 31. Each subassembly cancomprise a single transistor subassembly or a plurality of transistorsubassemblies being fabricated on a flexible substrate.

Referring to FIG. 1, a method 100 of using a non-self-aligned approachis shown for fabrication of a silicon-based thin film transistor on aflexible substrate, such as a plastic or polymeric substrate or a metalfoil substrate. In one embodiment, method 100 can comprise: 1)optionally depositing an insulating/smoothing film or barrier layer suchas silicon nitride or silicon oxide or a spin on glass; 2) depositing asilicon film using a suitable deposition process to form isolatedislands of silicon on the smoothing film if present, or directly ontothe flexible substrate; 3) optionally laser annealing the depositedsilicon film to improve the crystalline nature of the film, 4) dopingareas of the silicon islands to form source and drain electrodes whileleaving areas of the silicon undoped to form the channel of atransistor; 5) depositing a dielectric material such as silicon dioxide,silicon nitride, hafnium oxide or the like using a low temperatureprocess to form the gate dielectric while leaving the contact padsuncovered by the dielectric material; 6) depositing the gate electrode;and 7) and optionally encapsulating the TFTs with a protective layer.

At optional step 102 of method 100, a barrier layer 202 is deposited onsubstrate 200 to form subassembly 50, as depicted in FIG. 2. Substrate200 can comprise a flexible material such as, for example, a polymericmaterial, plastic material, paper, nonwoven, woven, stainless steelfoil, Al coated plastic film or any other suitable flexible material. Inone embodiment, substrate 200 comprises polyethylene terephthalate (PET)film. Substrate 200 can be provided in either web form or sheet form.Substrate 200 can comprise a thickness ranging from about 25 micron toabout 400 micron.

Barrier layer 202 functions to insulate the device, or transistor, fromthe substrate both electrically and thermally. In one embodiment,barrier layer 202 is a dielectric material, such as, for example,silicon nitride, silicon dioxide, a spin-on-glass or combinationsthereof. Barrier layer 202 can also act as a barrier material foroxygen, water or other gas phase species by reducing the permeation ratethrough substrate 200 to protect the fabricated transistor. Further,barrier layer 202 can act as a smoothing film to reduce the roughness ofsubstrate 200 resulting in a smoother interface between substrate 200and other materials. Additionally, barrier layer 202 can act as athermal barrier to protect the substrate from damage due to subsequentprocessing steps such as laser processing.

Barrier layer 202 can be deposited on at least one surface of substrate200 using any of a variety of suitable deposition processes, such as,for example, traditional printing techniques, chemical vapor deposition(CVD), plasma-enhanced CVD (PECVD), electron beam evaporation, sol gel,liquid phase deposition from a chemical bath and the like, and/or any ofa variety of suitable coating processes, such as, for example, diecoating (i.e. slot die), curtain coating, spray coating, roll coating,screen coating, and the like and combinations thereof. The use of a maskor masking layer can also be employed to form a patterned barrier layer.In one embodiment of the invention, barrier layer 202 comprises aspin-on material or ink that is deposited by traditional printingtechniques, such as, for example, flexography, screen printing, inkjet,and/or gravure to form a patterned barrier layer. Printing plates usedin the above printing processes can be formed by one or more methods asdescribed in U.S. Patent Application Publication No. 2008/0092377entitled “Patterned Printing Plates And Processes For PrintingElectrical Elements.” Such printing plates can result in higher qualitysurface morphology, and/or tighter tolerances of the printed design.

In one embodiment of the invention, barrier layer 202 is depositedhaving a thickness from about 500 nanometers (nm) or less. In someembodiments of the invention, barrier layer 202 is vacuum annealedbefore further processing.

Step 104 of method 100 can comprise depositing a thin undoped or lightlydoped silicon film upon barrier film 202 (if present) or directly onsubstrate 200. The silicon material can comprise amorphous silicon,polycrystalline silicon, or microcrystalline silicon, preferably of lowgas content (e.g. hydrogen, argon). Conventional deposition processescan be utilized. In one embodiment, microcrystalline silicon isdeposited using PECVD wherein a silicon-containing gas is introducedinto a vacuum chamber. A plasma is struck with the gas in the presenceof the substrate 200. This allows silicon atoms to be deposited andresult in a high quality, thin coating of silicon, such as, for example,of a thickness of about 200 nm or less. In an alternative embodiment, aphysical vapor deposition process such as sputter coating (DC) can beused to deposit amorphous silicon onto substrate 200 or barrier layer202 (if present). In yet another embodiment, a silicon ink can beprinted onto substrate 200 and/or barrier layer 202 with traditionalprinting techniques, such as, for example, jetting systems such as inkjet, screen printing, gravure, and flexography.

Referring to FIGS. 3A and 3B, in one embodiment, isolated patternedsilicon deposits 204 can be deposited on barrier layer 202 to formsubassembly 52, using a masking process, such as, for example, a shadowmask process. A conventional shadow mask process involves the use of apatterned structure (the shadow mask) wherein the pattern allowsmaterials to pass through the patterned structure in selected areas andblocks the passage of materials through the structure in other areas.The shadow mask is typically brought into close proximity with thesubstrate or in direct contact with the substrate, the material ofinterest is deposited, and the shadow mask is removed. This allows thematerial to be deposited on the substrate only where the material canpass through the shadow mask.

Another masking process includes a lift off process. The lift offprocess can include printing of a patterned masking layer on barrierfilm 202 or substrate 200, followed by deposition of silicon on themasking layer. A lift-off process is then used to remove the maskinglayer, leaving the silicon that is in direct contact with barrier layer202 and/or substrate 200 to create the patterned deposit 204.

Yet another masking approach for forming patterned Si structures on thesubstrate is to perform a blanket deposition of Si, deposit a maskinglayer by printing or another suitable deposition technique, and performa wet or dry etch of the Si. The masking layer would then be removedleaving the patterned Si on the substrate. Another approach is todeposit a blanket layer of Si and use laser ablation to pattern the Si.Deposits 204 can be patterned to include contacts 206. Examples ofsuitably-shaped deposits with contacts 206 are illustrated in FIGS. 4and 5. Interdigitated structures can also be created.

In one embodiment of the invention, barrier layer 202 is depositedhaving a thickness from about 500 nanometers (nm) or less. In someembodiments of the invention, barrier layer 202 is vacuum annealedbefore further processing.

Step 106 of method 100 comprises an optional annealing process toimprove the crystallinity of the silicon, and therefore the performanceof the resulting transistor is improved. In one embodiment, a laserannealing process is performed such as described in U.S. Pat. No.4,409,724 incorporated herein by reference.

Step 108 of method 100 comprises doping of selected areas of silicondeposit 204 by using a laser deposition process, such as, for example,the GILD process as described above to form subassembly 54 illustratedin FIGS. 6A and 6B. In the past, the GILD process was used on siliconwafers that were patterned with a masking layer. However, in the presentinvention, no such masking layer is needed as the areas of silicondeposit 204 to be doped can be precisely controlled by focus of thelaser. A gas, such as PF₅ or BF₃, is introduced into a vacuum chamber.The laser melts selected areas of the silicon deposit 204 and the dopantgas reacts with the liquid silicon. Further, atomic phosphorous or borondiffuses into the liquid silicon, and becomes physically trapped,occupying substitutional sites and being electrically active.

As depicted in FIGS. 6A and 6B, the GILD process or other suitableprocess can be used to dope selected areas of silicon deposit 204 tocreate doped areas 208 forming the source and the drain of thetransistor, while leaving areas of the silicon deposit 204 unchangedwith respect to dopant level to form a channel 210 of the transistor.N-type doping to form an NMOS circuit, P-type doping to form a PMOScircuit, or both can be used to form a CMOS circuit. If the channel 210is lightly doped with one type of dopant, the source and drain should bedoped with the opposite type of dopant.

Another approach that can be used to create doped regions of Si is toselectively or non-selectively deposit a “spin on dopant” material byprinting or other suitable deposition technique. Spin on dopants arecommercially available from, for example, Honeywell, and are used in thephotovolatics industry. A laser can then be used to heat and/or melt Siin selective areas and drive the dopant from the spin on dopant into theSi. Alternatively, the spin on dopant can be deposited selectivelyapplying it only to the source and drain regions, avoiding the channelregion, and a laser can be used to melt the silicon and drive the dopantinto the silicon only in areas where the spin on dopant is present. Ifdesired, both N-type and P-type dopants can be deposited prior to laserprocessing.

Another approach that can be used to create doped regions of Si is touse a plasma process to deposit a blanket layer of a dopant materialfollowed by laser heating and/or melting of the Si in selected areas todrive the dopant material into the Si. This can be done, for example, byusing a plasma and PF5 or BF3 gas.

In all of the processes described for creating doped Si regions, thelaser may be incident from either the front side of the substrate wherethe Si is deposited or the backside of the substrate if the laser lightis not absorbed by the substrate or the optional dielectric layer.

An alternative approach to using dopants for creating highly conductiveSi regions is to form a silicide by using an appropriate metal such astitanium, cobalt, nickel, platinum, tungsten or the like. Similardeposition and heating approaches can be used as described for thedopant based approach.

Step 110 of method 100 comprises depositing a gate dielectric layer 212.As depicted in FIGS. 7A and 7B, gate dielectric layer 212 covers atleast a portion of silicon deposit 204 such that contacts 206 areexposed, forming subassembly 56. Gate dielectric layer 212 can comprise,for example, materials such as silicon dioxide, silicon nitride, hafniumoxide and other suitable dielectric materials for fabricating TFTs andcombinations of these materials. Gate dielectric layer 212 can bedeposited by low temperature deposition means such as CVD, PECVD,sputtering (DC), liquid phase deposition or similar techniques. If suchtechnique is used, a masking layer, such as a shadow mask describedabove, can be used to pattern gate dielectric layer 212. A masking orsimilar technique should prevent the contact pads of the source anddrain electrodes from being covered by the dielectric material.

An alternative method of creating the patterned gate dielectric layer isto deposit a blanket layer of the dielectric material, and then deposita masking layer in selected areas and etch the gate dielectric materialin the areas that are not covered by the masking layer. The maskinglayer can then be removed leaving the patterned gate dielectricmaterial. Another approach is to deposit a blanket layer of the gatedielectric and use laser ablation to pattern the gate dielectric.

In one embodiment of the invention, gate dielectric layer 212 is equalto or less than about 300 nm thick.

In an alternative embodiment, gate dielectric layer 212 can be depositedusing a printing or coating technique, such as for example, a jettingsystem such as inkjet, gravure, and flexography, depending on thematerial used.

An optional annealing step can be used to improve the quality of thegate dielectric. This annealing may be done in a controlled atmospheresuch as a N₂, O₂, N₂+F₂, or combinations of these gases. The annealingstep may be carried out by conductive heating, IR based heating, laserirradiation of the gate dielectric causing its temperature to increase,laser irradiation of the underlying materials with conductive heattransfer to the gate dielectric causing its temperature to increase,inductive heating of the underlying materials, combinations of theseapproaches or other suitable means.

Step 112 of method 100 comprises depositing gate electrode 214 on gatedielectric layer 212 to form subassembly 58, as depicted in FIGS. 8A and8B. Gate electrode layer 214 can comprise a highly conductive material,such as, for example, gold, platinum, silver, nickel, titanium, cobalt,chromium, tungsten, aluminum, silicon, doped silicon, conductivepolymers, alloys of these materials, and any of a variety of highlyconductive materials and combinations thereof. Gate electrode 214 can bedeposited using any of a variety of techniques, such as CVD, PECVD, PVD,thermal transfer, printing processes such as, for example, gravure,jetting systems such as ink jet, flexography, lithography, screenprinting, and any of a variety of suitable printing techniques andcombinations thereof. In one embodiment of the invention, gate electrode214 is about 1000 nm thick or less. For optimal performance of theresulting transistor, gate electrode 214 should be printed in registerwith the source and drain electrodes such that gate electrode 214 ispositioned between them. This registration functions to minimize overlapand the associated capacitance. An optional laser trimming process maybe used to improve the size and shape of the gate electrode if needed bylaser ablating unwanted areas of the deposited gate electrode. If ablanket film is deposited, suitable patterning techniques can be used topattern the gate electrode such as laser ablation or a masking process.Optional interconnects can be deposited during this step as well.

Step 114 of method 100 comprises depositing an optional encapsulationlayer 218. Referring to subassembly 60 of FIGS. 9A and 9B, encapsulationlayer 218 functions to protect the resulting transistor 216.Encapsulation layer 218 can be deposited by any of a variety ofprocesses including a coating process such as slot die coating, curtaincoating, spray coating and the like, a printing process such asflexography, jetting systems such as inkjet, gravure, screen printing, adeposition process such as CVD, PVD, or PECVD, or a lamination process.Any combination of these processes can be used as well, and thetransistor can comprise multiple encapsulation layers 218. In oneembodiment, encapsulation layer 218 can comprise a polymer material,such as, for example, polyimide.

An annealing step may be performed after any or all of the depositionsteps or after the transistor structure is completely formed in order toimprove the transistor performance. This could be a thermal anneal in acontrolled environment or in the presence of a plasma such as a hydrogenplasma.

The above described method is intended to be exemplary, and notlimiting. Other deposition techniques, such as, for example, sputtercoating (DC) or other types of PVD, thermal transfer, spin coating, andthe like can be substituted for any of the deposition steps.

There are advantages in the performance of a TFT if the gate is wellaligned with the source and drain electrodes, as described in step 112of method 100 above. An alternative process to method 100 of achievingthis is to have a process flow where the gate is self-aligned. Referringto FIG. 10, in an alternative embodiment, a method 300 of using aself-aligned approach is shown for fabrication of a silicon-based thinfilm transistor on a flexible substrate, such as a plastic or polymericsubstrate or metal foil. In one embodiment, method 300 can comprise, inan order contemplated by one of skill in the art: 1) optionallydepositing an insulating/smoothing film such as silicon nitride orsilicon oxide; 2) depositing a silicon film using a suitable depositionprocess to form a blanket coating of silicon on the smoothing film ifpresent, or directly onto the flexible substrate; 3) optionally cleaningof the silicon layer; 4) laser annealing of areas of the silicon filmthat will become the “active” areas of the device to improve thecrystalline nature of the film; 5) deposition of a dielectric materialsuch as silicon dioxide at a low temperature process to form the gatedielectric layer over the silicon layer; 6) depositing a masking layerover only the crystallized area of the silicon film and curing of themasking layer if needed; 7) etching the “stack” comprising smoothingfilm, silicon (crystallized and uncrystallized areas), and gatedielectric to remove areas of the stack everywhere except under themasking layer; 8) stripping the masking layer without attacking thesubstrate and remaining stack; 9) depositing the gate electrode; 10)depositing a dopant material on the surface of one or more assemblies ofa plurality of assemblies; 11) laser doping the source and drain regionsof the silicon layer; 12) removing excess dopant; and 13) optionallyrepeating step 10 with the opposite type of dopant deposited on otherassemblies of the plurality of assemblies that is different from theassemblies with the other dopant material, followed by repeating steps11 and 12 to enable CMOS structures. Alternatively, if the dopants aredeposited in selected areas, both N-type and P-type dopants can bedeposited in step 10.

At optional step 302 of method 300, a barrier layer 402 is deposited onsubstrate 400 to form subassembly 62, as depicted in FIG. 11. Similarlyto step 102 as described above, substrate 400 can comprise a flexiblematerial such as, for example, a polymeric material, plastic material,paper, nonwoven, woven, stainless steel foil, Al coated plastic, or anyother suitable flexible material. In one embodiment of the invention,substrate 400 comprises polyethylene terephthalate (PET) film. Substrate400 can be provided in either web form or sheet form. Substrate 400 cancomprise a thickness ranging from about 25 micron to about 400 micron.

Barrier layer 402 functions to insulate the device, or transistor, fromthe substrate, both electrically and thermally. In one embodiment,barrier layer 402 is a dielectric material, such as, for example,silicon nitride, silicon dioxide, a spin on glass or combinationsthereof. Barrier layer 402 can also act as a barrier material foroxygen, water or other gas phase species by reducing the permeation ratethrough substrate 400 to protect the fabricated transistor. Further,barrier layer 402 can act as a smoothing film to reduce the roughness ofsubstrate 400 resulting in a smoother interface between substrate 400and other materials. Barrier layer 402 can also act as a thermal barrierto prevent damage to the substrate from subsequent processing steps suchas laser processing.

Barrier layer 402 can be deposited on at least one surface of substrate400 using any of a variety of suitable deposition processes, such as,for example, traditional printing techniques, chemical vapor deposition(CVD), plasma-enhanced CVD (PECVD), electron beam evaporation, sol gel,liquid phase deposition from a chemical bath and the like, and/or any ofa variety of suitable coating processes, such as, for example, diecoating (i.e. slot die), curtain coating, spray coating, roll coating,screen coating, and the like and combinations thereof. The use of a maskmay also be employed to form a patterned barrier layer.

In one particular embodiment, barrier layer 402 comprises a silicondioxide sputter film ranging in thickness from about 200 nm to about1000 nm.

In another embodiment of the invention, barrier layer 402 comprises apatterned spin-on glass material that is deposited by traditionalprinting techniques, such as, for example, flexography, screen printing,inkjet, and/or gravure. Printing plates used in the above printingprocesses can be formed by one or more methods as described in U.S.Patent Application Publication No. 2008/0092377 entitled “PatternedPrinting Plates And Processes For Printing Electrical Elements.” Suchprinting plates can result in higher quality surface morphology, and/ortighter tolerances of the printed design.

In some embodiments of the invention, barrier layer 202 is vacuumannealed before further processing.

In step 304, undoped or lightly doped silicon film 404 is deposited onbarrier layer 402 if present, or directly on substrate 400 to formsubassembly 64, as illustrated in FIG. 12. Silicon film 404 can compriseamorphous silicon, polycrystalline silicon, or microcrystalline silicon,preferably of low gas content (e.g., hydrogen, argon). Conventionaldeposition processes can be utilized. In one embodiment,microcrystalline silicon is deposited using PECVD wherein asilicon-containing gas is introduced into a vacuum chamber. A plasma isstruck with the gas in the presence of the substrate 400. This allowssilicon atoms to be deposited and result in a high quality, thin coatingof silicon, such as, for example, of a thickness of about 200 nm orless. In an alternative embodiment, a physical vapor deposition processsuch as DC sputter coating, pulsed DC sputter coating, or RF sputtercoating can be used to deposit amorphous silicon onto substrate 400 orbarrier layer 402 (if present).

In a particular embodiment, silicon film 404 has a thickness of about 50nm with a low content of gas, such as hydrogen and/or argon gas. Siliconfilm 404 can optionally be cleaned in step 305 either in vacuum oroutside of the vacuum system. An HF clean, RCA clean, or similar is asuitable cleaning technique for cleans performed outside of the vacuumsystem. A vapor HF or plasma based clean is appropriate for cleansperformed inside the vacuum system.

In an alternative embodiment illustrated in FIG. 12A, isolated patternedsilicon deposits 403 can be deposited on barrier layer 402 to formsubassembly 65 using a masking process, such as, for example, a shadowmask process. A conventional shadow mask process involves the use of apatterned structure (the shadow mask) wherein the pattern allowsmaterials to pass through the patterned structure in selected areas andblocks the passage of materials through the structure in other areas.The shadow mask is typically brought into close proximity with thesubstrate or in direct contact with the substrate, the material ofinterest is deposited, and the shadow mask is removed. This allows thematerial to be deposited on the substrate only where the material canpass through the shadow mask.

Another masking process includes a lift off process. The lift offprocess can include printing of a patterned masking layer on barrierfilm 402 or substrate 400, followed by deposition of silicon on themasking layer. A lift-off process is then used to remove the maskinglayer, leaving the silicon that is in direct contact with barrier layer402 and/or substrate 400 to create the patterned deposit 403.

Yet another masking approach for forming patterned Si structures on thesubstrate is to perform a blanket deposition of Si, to form blanket film404, deposit a masking layer by printing or another suitable depositiontechnique, and perform a wet or dry etch of the Si. The masking layerwould then be removed leaving the patterned Si on the substrate. Anotherapproach is to deposit a blanket layer of Si and use laser ablation topattern the Si. Deposits 403 can be patterned to include contacts.Interdigitated structures can also be created.

In yet another embodiment, a silicon ink can be printed onto substrate400 and/or barrier layer 402 with traditional printing techniques, suchas, for example, jetting systems, gravure, and flexography to form apatterned silicon deposits 403.

In step 306, a laser annealing process, similar to step 106 of method100, is performed to crystallize only areas 406 of silicon film 404 orsilicon deposits 403 that will be active areas of the resultingtransistor device to form subassembly 66, as depicted in FIGS. 13A and13B. Silicon film 404 is annealed to improve the crystallinity of thesilicon, and therefore the performance of the resulting transistor isimproved. In one embodiment, a laser annealing process is performed suchas described in U.S. Pat. No. 4,409,724, incorporated herein byreference.

Suitable lasers can include, for example, eximer, YAG, and copper vaporlasers. In one embodiment of the invention, fiducial markers can also becreated with the laser, or alternatively, printed, embossed, orlaminated on silicon film 404 or the substrate.

In step 308, gate dielectric layer 408 is deposited over bothcrystallized areas 406 and the remaining silicon film 404, to formsubassembly 68 as illustrated in FIG. 14. Gate dielectric layer 408 cancomprise, for example, materials such as silicon dioxide, siliconnitride, hafnium oxide and other suitable dielectric materials forfabricating TFTs and combinations of these materials. Gate dielectriclayer 408 can be deposited by low temperature deposition means such asCVD, PECVD, sputtering (DC and the like), liquid phase deposition orsimilar techniques, similar to the processes described in step 110 ofmethod 100, with or without masking layers. In one embodiment of theinvention, gate dielectric layer 212 is equal to or less than about 300nm thick. In a particular embodiment, gate dielectric layer 408comprises silicon dioxide deposited by PECVD at low temperatures.

Preferably, gate dielectric layer 408 comprises a high quality oxide. Insome embodiments of the invention, a cleaning step is performed tosilicon layer 404 prior to depositing gate dielectric layer 408. Thisclean can include a suitable procedure, such as, for example, an HFclean, followed by UV/O₂ or UV/O₂/F₂ process to form a thin, highquality SiO2 interface between Si and the deposited gate dielectric.This interface improves electrical performance between the Si and thedeposited gate dielectric.

An alternative method of creating the patterned gate dielectric layer isto deposit a blanket layer of the dielectric material, and then deposita masking layer in selected areas and etch the gate dielectric materialin the areas that are not covered by the masking layer. The maskinglayer can then be removed leaving the patterned gate dielectricmaterial. Another approach is to deposit a blanket layer of the gatedielectric and use laser ablation to pattern the gate dielectric.

In an alternative embodiment, gate dielectric layer 408 can be depositedusing a printing or coating technique, such as for example, a jettingsystem such as inkjet, gravure, screen printing, and flexography,depending on the material used.

An optional annealing step may be used to improve the quality of thegate dielectric. This annealing may be done in a controlled atmospheresuch as a N2, O2, N2+F2, or combinations of these gases. The annealingstep may be carried out by conductive heating, IR based heating, laserirradiation of the gate dielectric causing its temperature to increase,laser irradiation of the underlying materials with conductive heattransfer to the gate dielectric causing its temperature to increase,inductive heating of the underlying materials, combinations of theseapproaches or other suitable means.

In step 310, a masking layer 410, such as an etch mask, resist layer orphotomasking layer, is applied only over crystallized areas 406, to formsubassembly 70 as shown in FIG. 15. Masking layer 410 may be applied bytraditional printing techniques such as screen printing, jetting systemssuch as inkjet printing, gravure printing, flexography, or the like.Additionally, masking layer 410 may be applied by spray coating througha shadow mask or through the use of traditional photolithographytechniques. In various embodiments, masking layer 410 is cured using acuring process such as, for example, UV curing, thermal curing, electronbeam curing, and combinations thereof. By applying masking layer 410only over areas 406, a blanket exposure, such as to UV light, can beused to cure or expose the material of masking layer 410.

In step 312, an etching process is used to remove the stack comprisingof gate dielectric layer 408, silicon layer 406, and optional barrierlayer 402, everywhere except under masking layer 410, to formsubassembly 72 depicted in FIG. 16. Any suitable etching process, suchas, for example, wet etching, dry etching, etching pastes, and othersuitable etching techniques and combinations thereof may be used. In oneembodiment, step 312 comprises using an HF etch to etch dielectric layer408, followed by using KOH or NH₄OH to etch silicon layer 404, followedby an additional HF etch to etch barrier layer 402.

Following etching step 312, masking layer 410 is stripped in step 314,to form subassembly 74 as shown in FIG. 17. Depending on the materialused for masking layer 410, this could be done using a suitable solvent,or other similar techniques. A suitable process includes one that doesnot attack exposed substrate 400.

Referring to FIGS. 18A and 18B, gate electrode 412 is deposited on theresulting stack in step 316 to form subassembly 76. Gate electrode layer412 can comprise a highly conductive material, such as, for example,gold, platinum, silver, nickel, titanium, cobalt, chromium, tungsten,aluminum, silicon, doped silicon, conductive polymers, alloys of thesematerials, and any of a variety of highly conductive materials andcombinations thereof. Gate electrode 412 can be deposited using any of avariety of techniques, such as CVD, PECVD, PVD, thermal transfer,printing processes such as, for example, gravure, jetting systems suchas ink jet, flexography, lithography, screen printing, and any of avariety of suitable printing techniques and combinations thereof. Anoptional laser trimming process may be used to improve the size andshape of the gate electrode if needed by laser ablating unwanted areasof the deposited gate electrode.

If a blanket film is deposited, suitable patterning techniques can beused to pattern the gate electrode. In one embodiment, a blanket layerof silicon is deposited. Following this step a masking material isprinted and the silicon is etched everywhere except under the maskinglayer. The masking layer is then removed and the remaining siliconserves as the gate electrode. Optional interconnects can be depositedduring this step as well.

An optional protective layer (not depicted) is formed at optional step319 over gate electrode layer 412 to protect gate electrode layer 412from any additional processing, such as laser doping and the likeprocesses described in steps 320 through 324 infra. This protectivelayer protects gate electrode layer 412 from laser ablation, forexample, in a laser doping or GILD-type process. For example, if gateelectrode comprises a material other than silicon, such as a conductivemetal, an optional protective layer can protect the gate electrode fromlaser ablation when the silicon is undergoing doping processes.

In one alternative embodiment, portions of gate dielectric layer 408that are not positioned under gate electrode 412 are optionally etchedusing processes such as wet etch, etching pastes, and any other suitableetch process to form subassembly 78 as depicted in FIG. 19. In oneembodiment, exposed gate dielectric layer 408 is etched using an HF etchprocess. It is this step where gate electrode 412 acts as a mask toself-align gate with the regions that will act as source and drainelectrodes of the transistor.

In step 320, a dopant material, such as an N-type dopant or a P-typedopant, is deposited on the exposed surface of subassembly 78. In oneembodiment, this step is accomplished by blanket coating one or morelayers of boron or phosphorous by PECVD. The resulting transistor 80 isillustrated in FIG. 19A with doped areas 413.

Selected regions 413 of the device are then laser doped in step 322 toform the source and the drain, with an undoped region between themforming a channel. This is accomplished by melting silicon layer 404using the laser and driving the dopant material into the film. Analternative to the doping technique described above, is the use of a“spin on dopant” which is currently used in the photovoltaic industry.In this embodiment, a spin on dopant may be printed on subassembly 78and subsequently laser annealed. Additionally, N-type spin on dopantsmay be printed on a select group of subassemblies 78 and P-type spin ondopants may be printed on a different select group of subassemblies 78.Following the printing of both the N-type and P-type spin on dopants thefilms are laser annealed. When the gate electrode material is silicon,the gate electrode may be doped at the same time as the source and drainregions. Alternatively the GILD process can be used to selectively dopethe Si areas.

As discussed above, the optional protective layer (not depicted) isformed at optional step 319 over gate electrode layer 412 before eitherstep 320 or step 322 to protect gate electrode layer 412 from anyadditional processing, such as laser doping and the like processesdescribed in steps 320 through 324. This protective layer protects gateelectrode layer 412 from laser ablation, for example, in a laser dopingor GILD-type process. In an alternative embodiment, subassembly 78 isirradiated from the backside. This allows the barrier, silicon, anddielectric layers to absorb the energy of the laser, thereby acting as ashield for gate electrode layer 412.

Excess dopant is removed in step 324. This can be done by any suitablemeans, such as, for example, a water rinse, a wet clean usingappropriate chemicals, a heat step in vacuum to evaporate the dopant ora clean using gases in a vacuum system. Completion of steps 320 through324 results in a structure where the gate is automatically aligned withthe source and drain electrodes as the gate “shadows” the channel areaand dopant material is not in contact with the Si under the gateelectrode. In a matrix of TFTs, such as on a web, all or only some ofthe TFTs can be doped.

In some embodiments, at least one of the TFTs in a matrix is doped withN-type to create NMOS structures, and at least one other of the TFTs isdoped with P-type to create PMOS structures. In yet another embodiment,at least one TFT is doped with first either a P-type or N-type dopant,and then steps 320 through 324 are repeated in optional step 326 to dopeat least one other TFT with the other of either a P-type dopant or anN-type dopant, resulting in a CMOS structure. An additional cleaningprocess, such as an HF process, may be needed in between dopingsequences.

Further, an optional encapsulation layer can be added, similar to step114 of method 100 to protect the resulting transistors. Theencapsulation layer can be deposited by any of a variety of processesincluding a coating process such as slot die coating, curtain coating,spray coating and the like, a printing process such as flexography, adeposition process such as CVD, PVD, or PECVD, or a lamination process.Any combination of these processes can be used as well, and thetransistor can comprise multiple encapsulation layers. In oneembodiment, the encapsulation layer can comprise a polymer material,such as, for example, polyimide.

An annealing step may be performed after any or all of the depositionsteps or after the transistor structure is completely formed in order toimprove the transistor performance. This could be a thermal anneal in acontrolled environment or in the presence of a plasma such as a hydrogenplasma.

The sequence of steps of methods 100 and 300 as described above are forexemplary purposes only and other sequences contemplated by one ofordinary skill in the art are incorporated herein by reference. Forexample, in method 300, steps 302 through 324 can be completed in thesequence as described. In an alternative embodiment, for example, steps310, 312, and 314 can be performed after steps 302 and 304, followed bystep 308, then steps 316 through 324, with the option of repeating steps320 through 324, as described above. This approach can possibly minimizethe possibility of a short between the gate and channel of the TFT.

In yet another embodiment depicted in FIG. 20, a method 500comprises: 1) optionally depositing insulating/smoothing film on aflexible substrate; 2) depositing a silicon film on the optionalsmoothing film; 3) depositing a masking layer on the silicon film layer;4) etching the silicon and optionally etching the insulting/smoothingfilm; 5) stripping the masking layer and cleaning the silicon layer; 6)optionally laser annealing the silicon layer at selected areas; 7)depositing a gate dielectric material over the silicon layer; 8)depositing the gate electrode on the gate dielectric layer; 9) etchingthe gate dielectric layer that is not positioned under the gateelectrode; 10) optionally cleaning the silicon layer; 11) depositing adopant material on the surface of some subassemblies; 12) laser dopingareas of the silicon; 13) removing excess dopant; and 14) optionallyrepeating steps 11 through 13 with an opposite dopant material on otherassemblies.

More particularly, in optional step 502 of method 500, a barrier layeris deposited on a flexible substrate, as previously described in steps102 and 302 above. The substrate can comprise a flexible material suchas, for example, a polymeric material, plastic material, paper,nonwoven, woven, stainless steel foil, Al coated plastic or any othersuitable flexible material. The substrate can be provided in either webform or sheet form. The substrate can comprise a thickness ranging fromabout 25 micron to about 400 micron. In one embodiment of the invention,the barrier layer can comprise a dielectric material, such as, forexample, silicon nitride, silicon dioxide, spin on glass or combinationsthereof.

In step 504, an undoped or lightly doped silicon film is deposited onthe barrier layer if present, or directly on the substrate, similar tostep 304 described above. The silicon material can comprise amorphoussilicon, polycrystalline silicon, or microcrystalline silicon,preferably of low gas content (e.g., hydrogen, argon). Conventionaldeposition processes can be utilized. In one embodiment of theinvention, microcrystalline silicon is deposited using PECVD wherein asilicon-containing gas is introduced into a vacuum chamber. In analternative embodiment of the invention, a physical vapor depositionprocess such as DC sputter coating, pulsed DC sputter coating, or RFsputter coating can be used to deposit amorphous silicon onto thesubstrate or barrier layer if present. In yet another embodiment, asilicon ink can be printed onto the substrate and/or barrier layer withtraditional printing techniques, such as, for example, jetting systems,gravure, screen printing, and flexography.

In an embodiment, the silicon film has a thickness of about 50 nm with alow content of gas, such as hydrogen and/or argon gas. The silicon filmcan optionally be cleaned either in vacuum or outside of the vacuumsystem. An HF clean, RCA clean, or similar is a suitable cleaningtechnique for cleans performed outside of the vacuum system. A vapor HFor plasma based clean is appropriate for cleans performed inside thevacuum system.

In step 506, a masking layer can be applied similar to the material andtechniques described in step 310. In embodiments of the invention, themasking layer is cured using a curing process such as, for example, UVcuring, thermal curing, electron beam curing, and combinations thereof.

In step 508, the silicon layer and optionally the barrier film (ifpresent) is etched everywhere except those areas covered by thepatterned masking layer using processes and materials as described abovein step 312. Any suitable etching process, such as, for example, wetetching, dry etching, etching pastes, and other suitable etchingtechniques and combinations thereof may be used. In one exampleembodiment, a KOH or NH₄ 0H wet etch is used to etch the silicon layer,and an optional HF etch process is used to etch the barrier layer. Instep 510, the masking layer is then stripped using techniques asdescribed in step 314, and the silicon layer is optionally cleaned.

In optional step 512, the silicon layer is annealed, such as by a laserannealing process as described in step 306. The silicon film is annealedto improve the crystallinity of the silicon, and therefore theperformance of the resulting transistor is improved. In one embodiment,a laser annealing process is performed such as described in U.S. Pat.No. 4,409,724, incorporated herein by reference. Suitable lasers caninclude, for example, eximer, YAG, and copper vapor lasers. In oneembodiment of the invention, fiducial marks can also be created with thelaser, or alternatively, printed, embossed, or laminated on the siliconfilm or the substrate.

In step 514, the gate dielectric layer is deposited on the silicon layerusing techniques as described in step 308. The gate dielectric layer cancomprise, for example, materials such as silicon dioxide, siliconnitride, hafnium oxide and other suitable dielectric materials forfabricating TFTs and combinations of these materials. The gatedielectric layer can be deposited by low temperature deposition meanssuch as CVD, PECVD, sputtering, liquid phase deposition or similartechniques, similar to the processes described in steps 110 and 308,with or without masking layers. In an alternative embodiment, gatedielectric layer 408 can be deposited using a printing or coatingtechnique, depending on the material used. In some embodiments of theinvention, a cleaning step is performed to silicon layer prior todepositing gate dielectric layer. This clean can include a suitableprocedure, such as, for example, an HF clean, followed by UV/O₂ orUV/O₂/F₂ process to form a thin, high quality SiO2 interface between Siand the deposited gate dielectric. An optional annealing step may beused to improve the quality of the gate dielectric. This annealing maybe done in a controlled atmosphere such as a N2, O2, N2+F2, orcombinations of these gases. The annealing step may be carried out byconductive heating, IR based heating, laser irradiation of the gatedielectric causing its temperature to increase, laser irradiation of theunderlying materials with conductive heat transfer to the gatedielectric causing its temperature to increase, inductive heating of theunderlying materials, combinations of these approaches or other suitablemeans.

In step 516, the gate electrode is deposited on the gate dielectriclayer, using techniques and materials as described in step 316 above.The gate electrode layer can comprise a highly conductive material, suchas, for example, gold, platinum, silver, nickel, titanium, cobalt,chromium, tungsten, aluminum, silicon, doped silicon, conductivepolymers, alloys of these materials, and any of a variety of highlyconductive materials and combinations thereof. The gate electrode can bedeposited using any of a variety of techniques, such as CVD, PECVD, PVD,thermal transfer, printing processes such as, for example, gravure,jetting systems such as ink jet, flexography, lithography, screenprinting, and any of a variety of suitable printing techniques andcombinations thereof. An optional laser trimming process may be used toimprove the size and shape of the gate electrode if needed by laserablating unwanted areas of the deposited gate electrode. If a blanketfilm is deposited, suitable patterning techniques can be used to patternthe gate electrode. In one embodiment, a blanket layer of silicon isdeposited. Following this step a masking material is printed and thesilicon is etched everywhere except under the masking layer. The maskinglayer is then removed and the remaining silicon serves as the gateelectrode. Optional interconnects can be deposited during this step aswell.

In step 518, the gate dielectric layer is etched everywhere except inareas under the gate electrode, similar to etching step 312 describedabove. Portions of the gate dielectric layer that are not positionedunder gate electrode are etched using processes such as wet etch,etching pastes, and any other suitable etch process. In one embodiment,exposed gate dielectric layer is etched using an HF etch process. It isthis step where the gate electrode acts as a mask to self-align gatewith the regions that will act as source and drain electrodes of thetransistor.

Step 520 comprises an optional cleaning step as described supra.

In step 522, a dopant material, such as an N-type dopant or a P-typedopant, is deposited on the exposed surface of the subassembly. In oneembodiment, this step is accomplished by blanket coating one or morelayers of boron or phosphorous by PECVD.

The source and drain regions of the device are then laser doped in step524. This is accomplished by melting the silicon layer using the laserand driving the dopant material into the film. An alternative to thedoping technique described above, is the use of a “spin on dopant” whichis currently used in the photovoltaic industry. In this embodiment, aspin on dopant may be printed and subsequently laser annealed.Additionally, N-type spin on dopants may be printed on a select group oftransistor subassemblies and P-type spin on dopants may be printed on adifferent select group of transistor subassemblies. Following theprinting of both the N-type and P-type spin on dopants the films arelaser annealed. When the gate electrode material is silicon, the gateelectrode may be doped at the same time as the source and drain regions.Alternatively the GILD process can be used to selectively dope the Siareas.

An optional protective layer (not depicted) is formed over the gateelectrode layer before either step 522 or step 524 to protect the gateelectrode layer from any additional processing, such as laser doping andthe like processes described in steps 522 through 524. This protectivelayer protects the gate electrode layer from laser ablation, forexample, in a laser doping or GILD-type process. In an alternativeembodiment, the subassembly is irradiated from the backside in the laserdoping step 524. This allows the barrier, silicon, and dielectric layersto absorb the energy of the laser, thereby acting as a shield for thegate electrode layer.

Excess dopant is removed in step 526. This can be done by any suitablemeans, such as, for example, a water rinse, a wet clean usingappropriate chemicals, a heat step in vacuum to evaporate the dopant ora clean using gases in a vacuum system. Completion of steps 522 through526 results in a structure where the gate is automatically aligned withthe source and drain electrodes as the gate “shadows” the channel area.In a matrix of TFTs, such as on a web, all or only some of the TFTs canbe doped.

In some embodiments, at least one of the TFTs in a matrix is doped withN-type to create NMOS structures, and at least one other of the TFTs isdoped with P-type to create PMOS structures. In yet another embodiment,at least one TFT is doped with first either a P-type or N-type dopant,and then steps 522 through 526 are repeated in optional step 528 to dopeat least one other TFT with the other of either a P-type dopant or anN-type dopant, resulting in a CMOS structure. An additional cleaningprocess, such as an HF process, may be needed in between dopingsequences.

Further, an optional encapsulation layer can be added, similar to step114 of method 100 to protect the resulting transistors. Theencapsulation layer can be deposited by any of a variety of processesincluding a coating process such as slot die coating, curtain coating,spray coating and the like, a printing process such as flexography, adeposition process such as CVD, PVD, or PECVD, or a lamination process.Any combination of these processes can be used as well, and thetransistor can comprise multiple encapsulation layers. In oneembodiment, the encapsulation layer can comprise a polymer material,such as, for example, polyimide.

An annealing step may be performed after any or all of the depositionsteps or after the transistor structure is completely formed in order toimprove the transistor performance. This could be a thermal anneal in acontrolled environment or in the presence of a plasma such as a hydrogenplasma.

Referring to FIG. 21, in another embodiment, a method 600 utilizing aself-aligning approach for the fabrication of a silicon-based thin filmtransistor on a flexible substrate, such as a plastic or polymericsubstrate utilizing a patterned insulating film is illustrated. In oneembodiment of the invention, method 600 comprises: 1) depositing aninsulating film or barrier material, such as a patterned film, on aflexible substrate; 2) depositing a silicon film; 3) optionallydepositing a masking layer on the silicon film layer; 4) optionallyetching the silicon and optionally etching the insulting film; 5)optionally stripping the masking layer and cleaning the silicon layer;6) laser annealing the silicon layer and optionally cleaning the siliconlayer; 7) depositing a gate dielectric material over the silicon layer;8) depositing the gate electrode on the gate dielectric layer; 9)printing a masking layer; 10) etching the gate electrode layer and gatedielectric layer and removing the masking layer; 11) depositing a dopantmaterial on the surface of the subassembly; 12) laser doping areas ofthe silicon; 13) removing excess dopant; and 14) printing contacts tosource, drain and gate.

More particularly, in step 602 of method 600 of FIG. 21, a barrier layer702 is deposited on a flexible substrate 700 to form subassembly 80, asdepicted in FIG. 22. Substrate 700 can comprise a flexible material suchas, for example, a polymeric material, plastic material, paper,nonwoven, woven, stainless steel foil, Al coated plastic or any othersuitable flexible material. Substrate 700 can be provided in either webform or sheet form. Substrate 700 can comprise a thickness ranging fromabout 25 micron to about 400 micron.

In various embodiments, patterned barrier layer 702 is printed to form anon-contiguous pattered layer. Patterned barrier layer 702 can bedeposited by traditional printing techniques such as, for example,gravure, screen printing, jetting systems such as ink jet and/orflexography processes. In alternative embodiments, patterned barrierlayer 702 is deposited utilizing sputter deposition. In anotherembodiment, barrier layer 702 is deposited by spray coating through ashadow mask. In one embodiment, patterned barrier layer 702 can comprisea dielectric material, such as, for example, silicon nitride, silicondioxide, spin on glass, or combinations thereof. In alternativeembodiments, patterned barrier layer 702 is comprised of a spin on glassdeposited utilizing printing techniques discussed above instead of thetraditional spin on processes.

Patterned barrier layer 702 can comprise a thickness of about 0 to about2000 nm, more specifically about 250 to about 1000 nm, and morespecifically about 300 to about 1000 nm. Thus, in various embodiments,patterned barrier layer 702 applied in step 602 of method 600 of FIG. 21allows for better adhesion of the barrier layer to the substrateespecially during flexure of the substrate, effectively reducingdelamination. Further, patterned barrier layer 702 allows for increasedflexibility and is less prone to the cracking exhibited when utilizingalternative methods.

Printing plates used in the above printing processes to form patternedbarrier layer 702 can be formed by one or more methods as described inU.S. Patent Application Publication No. 2008/0092377 entitled “PatternedPrinting Plates And Processes For Printing Electrical Elements.” Suchprinting plates can result in higher quality surface morphology, and/ortighter tolerances of the printed design.

In some embodiments of the invention, barrier layer 202 is vacuumannealed before further processing.

However, it should be realized that in an alternative embodiment of theinvention, an unpatterned, or blanket barrier layer can deposited asdescribed in step 102 of method 100, step 302 of method 300, and step502 of method 500. For exemplary purposes only, barrier layer 702 isreferred to as a patterned barrier layer.

In step 604 of FIG. 21, an undoped or lightly doped silicon film 704 isdeposited on patterned barrier layer 702 to form subassembly 82 asdepicted in FIG. 23. Silicon film 704 may be undoped, lightly P-doped,or lightly N-doped. Silicon film 704 can comprise amorphous silicon,polycrystalline silicon, or microcrystalline silicon, preferably of lowgas content (e.g., hydrogen, argon). Conventional deposition processescan be utilized. In one embodiment, a physical vapor deposition processsuch as DC sputter coating, pulse DC sputter coating, or RF sputtercoating can be used to deposit amorphous silicon onto patterned barrierlayer 702. In another embodiment, microcrystalline silicon is depositedusing PECVD wherein a silicon-containing gas is introduced into a vacuumchamber. In a particular embodiment, silicon film 704 has a thickness ofabout 50 to about 100 nm with a low content of gas, such as hydrogenand/or argon gas.

In yet another embodiment, a silicon ink can be printed onto substrate700 and/or barrier layer 702 with traditional printing techniques, suchas, for example, jetting systems, gravure, screen printing, andflexography to form isolated silicon deposits.

In step 606 of FIG. 21, a patterned masking layer 706 can be applied toselective areas using materials and techniques similar to thosedescribed in step 310, to form subassembly 84 of FIG. 24. In embodimentsof the invention, masking layer 706 is cured using a curing process suchas, for example, UV curing, thermal curing, electron beam curing, andcombinations thereof. In some embodiments, patterned masking layer 706is printed on silicon film 704.

Known printing techniques such as for example, gravure, jetting systemssuch as ink jet, flexography, lithography, screen printing, and any of avariety of suitable printing techniques and combinations thereof. Invarious embodiments patterned masking layer 706 is about 5 μm thick;

in a particular embodiment patterned masking layer 706 is about 0.3 μmthick. Further, in various embodiments the patterned masking layerdefines the active areas of the device. For example, in one embodimentmasking layer 706 is printed to an area of about 200 μm by about 500 μm.In this way, the printing of patterned masking layer 706 allows for moreprecision in defining the required features of the process and allowsfor tighter tolerances in the described method 600 of FIG. 21.

In step 608 in FIG. 21, silicon layer 704 is etched everywhere exceptthose areas covered by patterned masking layer 706. Any suitable etchingprocess, such as, for example, wet etching, dry etching, etching pastes,and other suitable etching techniques and combinations thereof may beused. In one embodiment of the invention, step 608 comprises using a KOHor NH₄OH to etch silicon layer 604. Thus, in various embodiments, theadvantages of patterned barrier deposition as described in step 602 isrealized as patterned barrier layer 702 does not require a separateetching step from current etch step 608.

Following etching step 608, patterned masking layer 706 is stripped instep 610 of FIG. 21, to form subassembly 86 as depicted in FIG. 25.Depending on the material used for masking layer 706, this could be doneusing a suitable solvent, or other similar techniques. A suitableprocess includes one that does not attack exposed substrate 700.

It should be realized however, that if a patterned silicon layer isapplied, such as by printing techniques described above, steps 606through 610 are not needed or are optional.

In step 612 of FIG. 21, silicon layer 704 is annealed, such as by alaser annealing process as described in step 306. Silicon layer 704 isannealed to improve the crystallinity of the silicon, and therefore theperformance of the resulting transistor is improved. In one embodiment,a laser annealing process is performed such as described in U.S. Pat.No. 4,409,724, incorporated herein by reference. Suitable lasers caninclude, for example, eximer, YAG, and copper vapor lasers. In oneembodiment, fiducial markers can also be created with the laser, oralternatively, printed, embossed, or laminated on the silicon film orthe substrate.

In some embodiments of the invention, a cleaning step is performed tosilicon layer 704 prior to depositing gate dielectric layer 708. Thisclean can include a suitable procedure, such as, for example, an HFclean, followed by UV/O₂ or UV/O₂/F₂ process to form a thin, highquality SiO₂ interface between Si and the deposited gate dielectric.This interface improves electrical performance between the Si and thedeposited gate dielectric.

In step 614 of FIG. 21, a gate dielectric layer 708 is deposited onsilicon layer 704 using techniques as described in step 308, to formsubassembly 88 as depicted in FIG. 26. Gate dielectric layer 708 cancomprise, for example, materials such as silicon dioxide, siliconnitride, hafnium oxide and other suitable dielectric materials forfabricating TFTs and combinations of these materials. Gate dielectriclayer 708 can be deposited by low temperature deposition means such asCVD, PECVD, sputtering (DC), liquid phase deposition or similartechniques, similar to the processes described in steps 110 and 308,with or without masking layers. In an alternative embodiment, gatedielectric layer 708 can be deposited using a printing or coatingtechnique such as jetting systems like inkjet, gravure, or flexography,depending on the material used. In still other embodiments liquid phasedeposition (LPD) is used to deposit suitable dielectric material such asHFO₂ or SiO₂. In various embodiments dielectric layer 708 is betweenabout 0 and about 300 nm thick, in one embodiment dielectric layer 708is between about 0 and about 50 nm thick. In this way, step 614 ofmethod 600 of FIG. 21 creates a lower threshold voltage in the resultantdevice, which increases flexibility and efficiency in application andreduces power consumption.

In step 616 of FIG. 21, gate electrode layer 710 is deposited on gatedielectric layer 708, using techniques and materials as described instep 316 above, to form subassembly 90 as depicted in FIG. 27. Gateelectrode layer 710 can comprise a highly conductive material, such as,for example, gold, platinum, silver, nickel, titanium, cobalt, chromium,tungsten, aluminum, silicon, doped silicon, conductive polymers, alloysof these materials, and any of a variety of highly conductive materialsand combinations thereof. Gate electrode layer 710 can be depositedusing any of a variety of techniques, such as CVD, PECVD, PVD, thermaltransfer, printing processes such as, for example, gravure, jettingsystems such as ink jet, flexography, lithography, screen printing, andany of a variety of suitable printing techniques and combinationsthereof. In a particular embodiment, sputter deposition, such as DCsputter coating, pulse DC sputter coating, or RF sputter coating, isutilized to deposit gate electrode layer 710. In various embodiments,gate electrode layer 710 is between about 0 and about 1000 nm thick.More specifically, in a particular embodiment, gate electrode layer 710is between about 20 and about 200 nm thick.

In step 618 of FIG. 21, a masking layer 712 is deposited on gateelectrode layer 710 to form subassembly 92 as depicted in FIG. 28.Masking layer 712 can be deposited using any of a variety of printingtechniques such as, for example, gravure, jetting systems such as inkjet, flexography, lithography, screen printing, and any of a variety ofsuitable printing techniques and combinations thereof. In oneembodiment, a wet masking layer 712 is printed on gate electrode layer710. In another embodiment, standard photolithography processes areutilized to deposit and define masking layer 712. Alternatively, laserablation can be used in place of a masking layer. For example, using adirect write method, a laser is focused to selectively ablate gateelectrode layer 710 and optionally gate dielectric layer 708, such thatno masking layer is needed. In a projection system, a patterned glassmask is place over the gate electrode layer. A laser is generallyapplied to the entire area, and the laser ablates any of gate electrodelayer 710 and optionally underlying gate dielectric layer 708 wherelaser light can pass through the glass mask.

In step 620 of FIG. 21, gate dielectric layer 708 and gate electrodelayer 710 layers are etched to form subassembly or transistor assembly94 as depicted in FIG. 29. The layers can be etched using processes suchas wet etch, etching pastes, and any other suitable etch process. In oneembodiment, gate dielectric layer 708 and gate electrode layer 710 areboth etched using a wet etch process. It is this step where gateelectrode layer 710 acts as a mask to self-align gate with the regionsthat will act as source and drain electrodes of the transistor.

It is realized that if one or both of gate dielectric layer 708 and gateelectrode layer 710 are selectively deposited so as to form a patternedlayer, step 620 may not be needed.

In step 622 of FIG. 21, at least one dopant material 714, such as anN-type dopant or a P-type dopant, is deposited on the exposed surface ofsubassembly 94 to form subassembly 96 as depicted in FIG. 30. In oneembodiment of the invention, this step is accomplished by blanketcoating one or more layers of boron or phosphorous by PECVD. In anotherembodiment, dopant material 714 is applied by utilizing various printtechniques such as gravure, jetting system (e.g. ink jet), screenprinting or flexography or is spray deposited. In one embodiment, thedopant material is a spin on dopant that may be printed or deposited byother means. In one embodiment, a layer of dopant material 714 about 5μm thick is deposited, more particularly about 1 μm thick, and moreparticularly about 0.2 μm thick.

Alternatively in step 622 of FIG. 21, an N-type dopant, such as aspin-on dopant, may be deposited on at least one transistor, and aP-type dopant, such as a spin-on dopant, may be deposited on at leastone other transistor to form subassembly 96 as illustrated in FIG. 30.More typically, a plurality of transistors 94 a can have an N-typedopant deposited on the exposed surfaces and a plurality of transistors94 b can have a P-type dopant deposited on the exposed surfaces as ameans for creating CMOS circuits, as illustrated in FIG. 30 a.

Dopant material 714 is laser processed in step 624 resulting in dopingof the underlying silicon in the source, drain and gate regions of thetransistor but not in the channel region. This is accomplished bymelting silicon layer 704 and silicon layer 710 using the laser anddriving dopant material 714 into the film to form source 715 and drain717. There is no or little presence of dopant material in channel region719 of the device as the dopant material is not in intimate contact withthe silicon in channel region 719 and therefore has very low possibilityof diffusing into channel region 719. Alternatively the GILD process canbe used to selectively dope the Si areas.

In one example embodiment, phosphorous is used as the dopant material,and is doped up to about 1×10²¹ atoms/cm², for a sheet resistance ofabout 200 ohms. In another embodiment of the invention, boron is used asthe dopant material, and is dope up to about 2×10²⁰ atoms/cm² for asheet resistance of about 12,000 ohms.

Dopant material 714 is stripped in step 626 of FIG. 21 to formsubassembly 98 in FIG. 3 1. This can be done by any suitable means, suchas, for example, a water rinse, a wet clean using appropriate chemicals,a heat step in vacuum to evaporate the dopant or a clean using gases ina vacuum system. Completion of steps 622 through 626 results in astructure 716 where the gate is automatically aligned with the source715 and drain 717 electrodes as the gate “shadows” channel area 719. Ina matrix of TFTs, such as on a web, all or only some of the TFTs can bedoped.

In step 628 of FIG. 21, contacts to source, drain and gate are printedusing suitable materials known in the art. In various embodimentsmaterials such as gold, platinum, silver, nickel, titanium, cobalt,chromium, tungsten, aluminum, alloys of these materials and the like,may be used to form contacts.

In certain embodiments of method 600, at least one of the TFTs in amatrix is doped with N-type to create NMOS structures, and at least oneother of the TFTs is doped with P-type to create PMOS structures. Invarious embodiments, a dopant of the first type, such as N-type, isapplied in step 622 to a plurality of subassemblies, followed by adopant of a second type, such as P-type, applied in step 622 to aseparate plurality of subassemblies, before the laser processing step624 is performed. After the doping step 624 is performed, any excessdopant is removed in step 626.

In an alternative embodiment, a dopant of the first type, such asN-type, is applied in step 622 to a plurality of subassemblies, which isthen followed by laser processing step 624. A dopant of a second type,such as P-type, is applied in step 622 to a separate plurality ofsubassemblies, which is then followed by a second laser processing step624. Any excess dopant is removed in step 626 after each separate laserprocessing step, or after both laser processing steps are performed.

In this way, method 600 for creation of TFTs allows for flexibility inthe application and doping of the various dopant types to create PMOS,NMOS and CMOS structures. An additional cleaning process, such as an HFprocess, may be needed in between doping sequences.

Further, an optional encapsulation layer can be added, similar to step114 of method 100 to protect the resulting transistors. Theencapsulation layer can be deposited by any of a variety of processesincluding a coating process such as slot die coating, curtain coating,spray coating and the like, a printing process such as flexography,screen printing, gravure, or jetting systems such as inkjet, adeposition process such as CVD, PVD, or PECVD, or a lamination process.Any combination of these processes can be used as well, and thetransistor can comprise multiple encapsulation layers. In oneembodiment, the encapsulation layer can comprise a polymer material,such as, for example, polyimide.

An annealing step may be performed after any or all of the depositionsteps or after the transistor structure is completely formed in order toimprove the transistor performance. This could be a thermal anneal in acontrolled environment or in the presence of a plasma such as a hydrogenplasma.

Printing plates used in any of the above printing processes, such asprinting of any of the transistor components, and/or printing of maskinglayers for any of the etching or patterning processes, can be formed byone or more methods as described in U.S. Patent Application PublicationNo. 2008/0092377 entitled “Patterned Printing Plates And Processes ForPrinting Electrical Elements.” Such printing plates can result in higherquality surface morphology, and/or tighter tolerances of the printeddesign. Furthermore, traditional photolithography techniques can be usedin any of the etching steps described above.

Systems for accomplishing the above described methods and techniques caninclude any number and combination of tool sets as contemplated by oneof ordinary skill in the art.

In one embodiment, referring to any of steps 302 through 306 of FIG. 10,any of steps 502 and 504 of FIG. 20, or any of steps 602 and 604 of FIG.21 can be performed in a deposition system (hereinafter “System 1”),such as a single rotary drum comprising at least one deposition vacuumchamber, and a laser annealing chamber. In an exemplary embodiment, therotary drum system can comprise a first deposition chamber for sputtercoating the substrate with a barrier layer, a second deposition chamberfor sputter coating a silicon layer, and a laser annealing chamber forcrystallizing areas of the silicon layer.

In another embodiment, step 308 of FIG. 10, step 514 of FIG. 20, or step614 of FIG. 21 can be performed in a dielectric deposition system(hereinafter “System 2”) comprising at least one cleaning station, suchas an HF cleaning station, and a deposition chamber for depositing thegate dielectric layer in series. The cleaning and deposition processescan be performed in the same or separate systems.

In another embodiment, step 310 of FIG. 10, step 506 of FIG. 20, or step606 of FIG. 21 can be performed using a masking layer printer system(“System 3”). For example, such masking layer printer system cancomprise a single printing station such as an inkjet or gravure station.In another example, the printer system comprises at least two maskinglayer printing stations in series to minimize the possibilities ofpinholes in the masking layer material. Optional cure stations can beplaced in masking layer printer systems, such as after the singleprinting station, or between and after the plurality of printingstations in the multiple printing station system.

In another embodiment, steps 312 and 314 of FIG. 10, steps 508, 510,and/or 518 of FIG. 20, or steps 608 and 610, and/or 620 can be performedin an etching system (“System 4”) comprising at least one etchingstation, followed by an optional masking layer stripping station. Theetching stations can comprise wet etching, dry etching, etching pastes,and combinations thereof. In one example, an HF etching station isfollowed by a KOH etching station, followed by a second HF etchingstation, followed by a solvent-based masking layer stripping station, toaccomplish a three-step etching process as described above in step 312of FIG. 10. One or more optional rinsing stations and/or one or moreoptional drying stations can be placed between and after each etchingstation and masking layer stripping station.

In another embodiment, step 316 of FIG. 10, step 516 of FIG. 20, or step616 of FIG. 21 can be accomplished via a gate electrode printer system(“System 5”). Such system can comprise, for example, a single printingstation, such as an inkjet or gravure station, followed by one or morecuring stations. Alternatively, a system resembling System 3 can also beused.

In another embodiment, step 518 of FIG. 20 or step 620 of FIG. 21 can beperformed using either a single station wet bath (“System 6”) or can becombined with the etching and doping system described as follows. Steps320 and 322 of FIG. 10, steps 522 and 524 of FIG. 20, or steps 622 and624 of FIG. 21 can be combined in a single etching and doping system(“System 7”) comprising a first etching station such as an HF etchingstation, a dopant deposition station, such as a PECVD station, followedby one or more laser doping stations.

In another embodiment, step 324 of FIG. 10, step 526 of FIG. 20, or step626 of FIG. 21 can be accomplished using a single station wet bath withoptional drying (“System 8”). Alternatively, a system resembling theSystem 4 can be used described above.

In another embodiment, the repetition of steps 320 through 324 of FIG.10, steps 520 through 526 of FIG. 20, or steps 622 through 626 of FIG.21 can be performed in System 7 itself, or as its own system. Theadvantages of having its own system include throughput advantages (forweb-based operations, no need to perform multiple passes), and to avoidcross-contamination of dopant types when using opposite dopant types.

The methods of the present invention can be accomplished in a singleapparatus comprising at least Systems 1, 2, 3, 4, and 7.

The invention therefore addresses and resolves many of the deficienciesand drawbacks previously identified. The invention may be embodied inother specific forms without departing from the essential attributesthereof; therefore, the illustrated embodiments should be considered inall respects as illustrative and not restrictive.

1. A method for forming self-aligned silicon thin film transistorscomprising: providing a generally flexible substrate; depositing abarrier material on at least a portion of the flexible substrate;forming at least one silicon assembly on at least a portion of thebarrier material; cleaning the at least one silicon assembly using acleaning technique including UV/O₂, UV/O2/F2, wet clean, andcombinations thereof, thereby forming a SiO₂ interface on at least aportion of the at least one silicon assembly; depositing a dielectricmaterial on a least a portion of the SiO₂ interface to form a gatedielectric, wherein the dielectric material can be deposited byutilizing a low temperature technique including chemical vapordeposition (CVD), plasma-enhanced CVD (PECVD), sputter deposition, DCsputter deposition, pulsed DC sputter deposition, RF sputter deposition,electron beam evaporation, liquid phase deposition, printing andcombinations thereof; depositing a gate electrode material on at least aportion of the gate dielectric; selectively patterning the gateelectrode material and the dielectric material to form at least onetransistor assembly; selectively doping areas of the at least onetransistor assembly thereby forming a source and a drain; and removingexcess doping material to form a self-aligned silicon thin filmtransistor.
 2. The method of claim 1, wherein the barrier material isdeposited utilizing a low temperature technique selected from the groupconsisting of chemical vapor deposition (CVD), plasma-enhanced CVD(PECVD), sputter deposition, DC sputter deposition, pulsed DC sputterdeposition, RF sputter deposition, electron beam evaporation, liquidphase deposition, printing and combinations thereof.
 3. The method ofclaim 2, wherein the barrier material is deposited by a printingtechnique selected from the group consisting of flexography, a jettingsystem, gravure, screen printing, and combinations thereof, to form apatterned barrier layer.
 4. The method of claim 1, wherein the gateelectrode comprises a highly conductive material selected from the groupconsisting of gold, platinum, silver, nickel, titanium, cobalt,chromium, tungsten, aluminum, silicon, doped silicon, conductivepolymers, alloys of these materials, and combinations thereof.
 5. Themethod of claim 4, wherein the gate electrode comprises doped silicon.6. The method of claim 1, wherein the gate electrode is deposited bychemical vapor deposition (CVD), plasma-enhanced CVD (PECVD), PVD, DCsputter deposition, pulsed DC sputter deposition, RF sputter deposition,electron beam evaporation, thermal transfer or printing.
 7. The methodof claim 6, further comprising: patterning the gate electrode byprinting a masking layer over the gate electrode material to form maskedareas and unmasked areas, etching the gate electrode material in theunmasked areas, and removing the masking layer, thereby forming apatterned gate electrode.
 8. The method of claim 1, selectively dopingareas of the at least one transistor assembly comprises: introducing afirst dopant material to a first portion of a first transistor assemblyof the at least one transistor assembly; introducing the first dopantmaterial to a second portion of the first transistor assembly; laserannealing the first transistor assembly such that the first dopantmaterial is transferred into the first portion to form the source, andthe first dopant material is transferred into the second portion to formthe drain wherein a third portion of the first transistor assembly ispositioned between the first portion and the second portion and underthe gate electrode layer such that the first dopant material is notintroduced into the third portion, thereby forming a channel; andremoving any excess first dopant material.
 9. The method of claim 8,wherein the first dopant material is introduced to the first portion andthe second portion simultaneously, and then the first portion and secondportion are laser annealed to simultaneously form the source, the drain,and the channel.
 10. The method of claim 1, wherein selectively dopingareas of the at least one transistor assembly includes: depositing afirst dopant material on at least one of the at least one transistorassemblies, and laser annealing to dope the source, the drain, and thegate electrode.
 11. The method of claim 10, wherein the first dopantmaterial is blanket deposited on the at least one of the at least onetransistor assemblies, and the first dopant material is selectivelylaser annealed to dope the source, the drain, and the gate electrode.12. The method of claim 10, wherein the first dopant material isselectively deposited on the at least one of the at least one transistorassemblies, and the first dopant material is generally annealed to dopethe source, the drain, and the gate electrode.
 13. The method of claim10, wherein the at least one transistor assembly comprises at least thefirst transistor assembly and a second transistor assembly, and whereinselectively doping areas of the at least one transistor assembly furthercomprises: depositing a second dopant material on a second transistorassembly of the least one transistor assembly, and laser annealing thesecond transistor assembly to dope the source, the drain and the gateelectrode.
 14. The method of claim 8, wherein the first dopant materialis introduced to the first transistor assembly by selectively printingthe first dopant material to form the first portion and the secondportion, and the first transistor assembly is generally laser annealed.15. The method of claim 8, wherein the at least one transistor assemblycomprises at least the first transistor assembly and a second transistorassembly, the method further comprising: introducing a second dopantmaterial to a first portion of the second transistor assembly;introducing the second dopant material to a second portion of the secondtransistor assembly; laser annealing the second transistor assembly suchthat the second dopant material is transferred into the first portion toform a source, and the second dopant material is transferred into thesecond portion to form a drain wherein a third portion of the secondtransistor assembly is positioned between the first portion and thesecond portion and under the gate electrode layer such that the seconddopant material is not introduced into the third portion, therebyforming a channel; and removing any excess second dopant material. 16.The method of claim 15, wherein laser annealing is performed after boththe first dopant material and the second dopant are introduced to thefirst transistor assembly and second transistor assembly respectively.17. The method of claim 15, wherein the first dopant material isintroduced to the first transistor assembly and laser annealed beforethe second dopant material is introduced and laser annealed to thesecond transistor assembly.
 18. The method of claim 15, wherein at leastone of the first and second dopant materials is introduced to the atleast one transistor assembly by a printing technique selected from thegroup consisting of a jetting system, gravure, flexography, screenprinting, and combinations thereof.
 19. The method of claim 15, whereinthe first dopant material comprises an N-type dopant, and the seconddopant material comprises a P-type dopant.
 20. The method of claim 1,wherein the barrier material comprises a spin-on glass materialpatterned by a printing technique selected from the group consisting ofa jetting system, gravure, flexography, screen printing, andcombinations thereof.
 21. The method of claim 1, wherein forming atleast one silicon assembly comprises: depositing a silicon film on atleast one of the flexible substrate and the barrier material; cleaningthe silicon film; and laser annealing the silicon film.
 22. The methodof claim 21, further comprising: selectively printing a masking layermaterial over a portion of the silicon film to define masked areas andunmasked areas of the silicon film; etching the unmasked areas ofsilicon film; and removing the masking layer material to form the atleast one silicon assembly.
 23. The method of claim 1, furthercomprising: printing contacts to the source, gate electrode, and thedrain.
 24. The method of claim 1, further comprising: depositing aprotective layer over the gate electrode before selectively doping.